Strange. I can't find "Christ's Epistle to the Presbyterians" in my Bible. Protestants accuse the Pope of a lot of things, but the Pope doesn't write encyclicals that claim to be speaking the new and inspired words of Our Lord.
And my understanding of Protestant ecclesiology is that "Christ's Church" is invisible, and not to be confused with any earthly organization, which the PCUSA certainly is.
In case anyone thinks I'm being snarky toward Protestants in general, please keep in mind that the PCUSA has had abortion coverage in their insurance package for clergy for many years. To me, calling an apostate organization which enshrines baby killing as a "healthcare" option for their clergy ... calling them "Christ's Church" is objectively blasphemous.
Marcus Grodi, I believe, was a PCUSA pastor. When he was deciding it was time to bail, one of his PCUSA friends tried to argue him out of it, saying, "But, Marcus, you can't leave the Church!" Grodi blurted out, "If staying in the Church is so important, why did we leave the Church in the first place?!?" The PCUSA is way beyond any "reforming".
FYI Campion, the PCUSA being apostate doesn’t make the Pope the leader of the truth squad. Of course, not many it any in the PCUSA have theological issues with Rome.